A cellular or trunked communication system is one in which mobile or portable user terminals, such as mobile telephones or portable radios, herein collectively referred to as ‘mobile stations’, can communicate via a network infrastructure which generally includes a plurality of fixed base stations (base transceiver stations). Each base station has one or more transceivers which serve mobile stations in a given region or area, known as a ‘cell’ or ‘site’, by radio communication. The cells of neighbouring base stations are often overlapping.
Generally, it is desirable for the mobile stations to be served by the base station which can provide the best service which includes good signals to and from the mobile station. Since mobile stations can move from one region to another it is known for the mobile stations to monitor signals from different base stations, to operate a procedure to determine which base station can best serve the mobile station, to determine whether it would be worthwhile to switch from a current serving base station to another one and if appropriate to carry out such a switch. In the art, the procedure to monitor for and to make determinations regarding a possible switch is known as a ‘cell re-selection’ procedure. The procedure to carry out a switch using such a determination is known as ‘handover’ or ‘handoff’.
Such a cell re-selection procedure is operated in various cellular or trunked systems, e.g. in TETRA systems, i.e. those that operate in accordance with TETRA (Terrestrial Trunked Radio) standards defined by the European Telecommunications Institute (ETSI). In TETRA systems, each base station continuously broadcasts to mobile stations served by that station information relating to neighbour base stations which could serve the mobile stations. The information is broadcast on a control channel. The information includes channel frequencies and other data which allows each mobile station easily to search for other base stations in a scanning operation. Each mobile station runs an algorithm which produces and records link measurements between the mobile station and the neighbour base stations. These measurements are based on recorded received signal strength (energy) values. The algorithm uses the measurements plus some service parameters (such as local site trunking availability, subscriber class served, class of security available, and level of cell services available) to build a list of selected neighbour cells in a preferred order. When the mobile station determines that the link with the current serving base station should be exchanged with a better link it abandons the link with current serving base station and undergoes handover to the selected neighbour base station which is top of the compiled list.
In the known procedure for TETRA systems, the decision as to whether the mobile station should undergo cell handover to a selected neighbour base station is based on a comparison between the received signal strength ESRV (on an energetic basis) from the serving base station and the received signal strength ENGB from the selected neighbour base station. Two conditions have to be met using these measurements. Firstly, the measured value of ESRV has to be less than a pre-determined threshold. Secondly, the measured value of ENGB has to be greater than ESRV by an amount equal to or greater than a pre-determined reference difference known as the ‘re-selection hysteresis’. Generally, the hysteresis is used by the system to avoid situations in which the mobile station flips its attachment back and forth between two base stations as its serving base station. The pre-determined threshold and the re-selection hysteresis are different, in the two cases in which (i) the mobile station is in an idle mode, i.e. not actively participating in a call, and (ii) the mobile station is in a call-active mode in which it is actively participating in a call. The pre-determined threshold and the re-selection hysteresis are constants for each mode but are greater in mode (i) than in mode (ii). The reason for this is that when a call is established it is considered better to maintain it with marginal link conditions rather than interrupt the call to allow handover to take place.